Liquid crystal display devices utilizing response of liquid crystals to electric field which have heretofore been developed and put into practice include those using so-called nematic liquid crystal compounds which can be categorized in DSM, TN, STN types, etc., or compositions containing these compounds.
However, those using nematic liquid crystals have a defect that their response time is very long, e.g., several milliseconds, and hence, it is said that attempts to increase in the size of the display screen has been reaching a limitation. Since the response of a nematic liquid crystal to an applied electric field is fundamentally based on anisotropy of dielectric constant of the liquid crystal molecule, its driving torque is small and response time is long.
Among the conventional nematic liquid crystal devices, chiral smectic liquid crystals developed by R.B. Meyer et al. show ferroelectricity and enable high speed response on the order of several micro seconds utilizing a large driving torque as expressed by direct interaction PsXE between spontaneous polarization (Ps) and applied electric field (E) (J. Physique, 36, L-69 (1975)). These chiral smectic liquid crystals, which are called ferroelectric liquid crystals, exhibit their function in a series of smectic phases tilted away from a smectic layer normal. Recently, a vigorous development has been made on, among others, chiral smectic C phase (hereinafter, referred to as "S.sub.c *" for short), which is appraised of superiority in practice because of its low viscosity.
As a typical example of a display device utilizing a ferroelectric liquid crystal, N.A. Clark and S.T. Lagerwall (Japanese Patent Application Laid-open No. 107216/1981; U.S. Pat. No. 4,367,924) proposed a liquid crystal display device using a liquid crystal having bistability, in which the spiral axis is parallel and layers are perpendicular to the substrate, and the spiral structure is made to disappear to generate a stabilized polarized domain on the surface, and orientation of the molecules is changed by polarity of the applied electric field.
However, it has been clarified that in spite of development of several thousands kinds of ferroelectric liquid crystal compounds and compositions having a Sc* phase, there are still problems which follow:
(1) There exists no clear, sharp direct current threshold, and dynamic driving is difficult; PA1 (2) There occurs incomplete switching or seizing due to presence of spontaneous polarization; PA1 (3) The orientation of the compounds is susceptible to mechanical impacts and tends to be readily broken. PA1 (4) There occurs decrease in contrast ratio due to twisted orientation called twist or deformation of smectic layer called chevron structure; and so on. PA1 (1) Orientation is easy and the device has a function of self-restoring defects; PA1 (2) There exist a clear direct current threshold and hysteresis; PA1 (3) There occurs no seizing or turbulence in orientation due to spontaneous polarization in the absence of electric field; and so on. PA1 1) Most of them have a ring structure such as biphenyl, naphthyl, dioxolanephenyl, cyclohexylphenyl or phenylpyrimidyl as a skeleton, and all but one exception (the compound disclosed in Japanese Patent Application Laid-open No. 218337/1991) contain three or more aromatic or aliphatic rings, and hence, have high liquid crystal temperature ranges in which they exhibit tristability (Jisedai Ekisho Disupurei to Ekisho Zairyo (Liquid Crystal Display of Next Generation and Liquid Crystal Materials), 287-336 (1992), CMC Corporation). PA1 2) Most of them have ester bonds as bonding between ring structures. Liquid crystals having ester bonds are known to generally have high viscosity, and for this reason the conventional compounds are considered to have long response time for tristable switching which involves generation and growth of domains. PA1 3) Although methylene oxide bonds instead of ester bonds have been studied with view to reduction of viscosity, the use of methylene oxide bonds deteriorates liquid crystal property and fails to give a broad liquid crystal range.
Therefore, there has been a demand for liquid crystal display devices which utilize effectively large direct interaction between spontaneous polarization and applied electric field and have high speed response to the applied field. However, no practically usable liquid crystal of such a type has been realized.
In 1988, A.D.L. Chandani et al. proposed a switching having tristability which utilizes a third state and two uniform states using 4-(1-methylheptyloxycarbonylphenyl) 4'-octylbiphenyl -4-carboxylate (MHPOBC) (Jpn. J. Appl. Phys., 27, L729 (1988)).
Recently, it has been found that the third state is an antiferroelectric chiral smectic phase, and tristable switching is phase transition between an electric field induced ferroelectric phase and an antiferroelectric phase.
Tristable switching differs from the aforementioned bistable switching of ferroelectric liquid crystal in:
However, few tristable switching liquid crystal compound (antiferroelectric liquid crystal) hitherto found are practically usable since they have defects which follow.
Therefore, it has been desired to develop liquid crystal compound and composition obviating the aforementioned defects and exhibit excellent tristability.